This invention relates to heat exchangers and is more particularly directed to heat exchangers in which a refrigerant fluid flows through the tubes and evaporates or condenses to accept heat from or give off heat to a coolant fluid in contact with the exterior of the tubes. The present invention is more specifically concerned with heat transfer tubes that have an internal rib enhancement, either with or without an external fin enhancement, and is also concerned with an improved method for making such tubing.
In the evaporator portion of certain refrigeration or air conditioning systems, a coolant fluid such as water passes through a chamber containing a number of tubes through which a refrigerant liquid is fed. The cooling fluid contacts the exterior of the tubes, and heats a refrigerant liquid in the tubes to evaporate it. The change of state of the refrigerant from liquid to vapor lowers the temperature of the coolant liquid. The internal configuration of the tubing is important in determining its overall heat transfer characteristics, and hence in determining the efficiency of the system. With evaporator tubing that has an internal rib enhancement, the evaporation takes place from a thin liquid film layer in contact with the internal surface, i.e., the sides and tips of the fins and the grooves between successive fins. An internal enhancement in the form of spiral or helical ribs causes swirling of the flowing refrigerant in the tube. This induces some turbulence, which breaks up laminar flow and thus also prevents any insulating barrier layer of vapor from forming on the interior surfaces of the tube.
Tubes that have an internal and/or an external enhancement are described, for example, in the commonly-assigned U.S. Pat. No. 4,425,696. That patent is directed to an evaporator tube configuration. Other finned tubes for heat transfer are described in U.S. Pat. Nos. 4,059,147 and 4,438,807.
In the tube finning machine employed in the production of this tubing, a grooved cylindrical mandrel within the tube produces the internal rib, while a tool gang of discs carried on a tool arbor produces a fin convolution on the exterior of the tubing. The force of the gang of discs on the metal tubing and against the mandrel causes the metal of the tubing to flow up between the discs to form the fins and down into the mandrels grooves to form the ribs. The external fins can be rolled over or smoothed by using a smooth disc.
Typically, a 5/8 inch heat exchanger tube has a starting blank wall thickness of 0.038 inch. The rib height is typically 0.020 to 0.030 inches, and there are about thirty internal ribs at a helix angle of thirty degrees.
It was desired to decrease the amount of materials required for the heat transfer tubes but without sacrifices of performance. In other words, it was desired to use thinner-walled blanks than the usual 0.038 inch-walled tubing, so that less copper would be required, or else a higher grade of copper could be employed without an increase in price. However, the standard mandrel-and-disc gang method of tube enhancement tended to weaken the tubes if the walls were much thinner than 0.038 inches. This is now believed to occur because the ribs were too high and the tube was worked too much. Thus the tendency to crack or split became unacceptably high.
Current techniques for tube enhancement involve ribbing and/or finning the entire tube, from one end to the other. When the tube is inserted into a tube sheet, it is typically secured by flaring or working the metal tube wall outwards into the circular collar of the tube sheet opening. After the metal wall has been once worked, i.e., by creating the internal enhancement, there is a tendency to flake or crack when the tube end is worked a second time. As a result, there is often an increased tendency to leak and a higher failure rate, if the tubes have an internal or external enhancement on its entire length.